Intaglio forms are conventionally prepared by mechanically engraving or chemically etching an array of dots into a printing surface of a flat or cylindrical platen, the dots having the shape of wells or pits which are filled with ink to be transferred to a sheet to be printed. The pits vary in diameter and/or depth to produce different tonal values, i.e. shades of gray or other color used for printing. In photoengraving, a positive replica of the copy to be reproduced is projecting through a halftone screen upon a pigment carrier, such as a sensitized gelatin film known in the art as carbon tissue, to create a latent image consisting of a multiplicity of discontinuous elemental areas subjected to different degrees of illumination. (The halftone screen may also be imaged upon the pigment carrier in a separate operation.) The pigment carrier, placed on a printing platen such as a copper cylinder, is washed with water to develop the latent picture and is thus tranformed into a masking layer enabling selective etching of the metallic substrate through its pattern of thickness variations.
A continuous-tone positive used for illuminating the pigment carrier is generally obtained by taking a negative of the original graphic material and photographically inverting that negative. Both the right-reading halftone negative and the laterally reversed halftone positive usually require retouching, which is a time-consuming operation particularly when several negatives and positives are required for multicolor printing.
More recent developments include the photoelectric scanning of the copy to be reproduced and the conversion of the resulting signals, with the aid of a computer, into binary pulses which are subsequently recorded on a film as a two-dimensional dot pattern to be used for producing the latent image on the pigment carrier. For this purpose, the film is swept by a laser beam which is modulated by the binary pulses read out from a data store in the computer, possibly with intermediate storage of these pulses in a memory. The digital coding of the dot pattern results in a sharper outline for the dots so as to make the halftone image less sensitive to fluctuations in developing time and temperature.
The intaglio process provides a high degree of contrast and a wide range of tonal values capable of forming colorful, high-quality reproductions of all types of paintings and original photographs. This technique is therefore widely utilized for art books, picture magazines, mail-order brochures and the like. A drawback, aside from the aformentioned laborious retouching operations, is the lack of a quick and inexpensive way of making advance proofs, such as those obtainable with planographic processes, which would enable the operator to correct or otherwise modify the copy before the etching step.